Large out-of-plane translation and high output force microactuators have a wide range of applications in adaptive optics and in micro robotics. In adaptive optics, they are used for auto-focus [1] and Optical Image Stabilization (OIS) in miniature cameras [2] and deformable micromirrors [3]. For the auto-focus in phone cameras application, the actuator is required to translate a lens of 3 mg in mass along the optical axis for 80 μm [4]. The OIS in phone cameras requires a rotation of a lens barrel of 45 mg mass [2] for 1° about two axes in order to cancel any effects hand-shaking has on the images and recorded video. In micro robotics, large stroke and high output force actuators are used in micro assembly systems and microgrippers [5][6].
Different micro actuation methods are in use. These include electromagnetic, piezoelectric, and electrostatic microactuators. The electromagnetic actuators provide large stroke and high output force; nevertheless, they are known to have a number of disadvantages such as high power consumption and large size [7][4]. Although piezoelectric actuators provide high output force, they are sensitive to temperature and are difficult to fabricate[1][8]. Electrostatic actuators offer high speed response, low power consumption, and small size [9][7][4]. However, it is challenging to design electrostatic actuators that can simultaneously provide high output force, large out-of-plane stroke and while maintaining a low voltage [10].
The electrostatic actuators that provide an out-of-plane stroke include parallel plate and Vertical Comb-Drive (VCD) actuators. The former suffers from the pull-in effect which limits the vertical stroke of the actuator to one third of the initial gap between the plates [9]. The latter could be divided into two types: rotational and translational (piston-style) VCD actuators.
In rotational comb-drive actuators (including staggered and angular VCD actuators), the original motion of the rotor is a rotation, thus providing a rotational out-of-plane stroke; however, these actuators often utilize mechanical amplification mechanisms, such as levers, to enlarge the stroke as well as to transform the original rotary motion of the rotor into a translation of the load. Due to the motion amplification and transformation, the output torque of the actuator acting on the load is generally lower than the one generated. Different designs of rotational VCD actuators have been developed. For example, a rotational VCD actuator developed by V. Milanovic et al. [11] achieved a vertical deflection of 60 μm (corresponding to an angle of rotation of 20°) at 150 V. It utilizes a cantilever for mechanical amplification. Li et al. [12] developed a rotational (staggered) VCD actuator that achieved a vertical deflection of 180 μm at 35 V. A plate was attached to the free comb of the actuator to achieve the large rotational stroke while reducing the output torque.
U.S. Pat. No. 8,358,925 B2 [13] discloses an invention of a rotational comb-drive actuator that is used to translate a lens along the optical axis (z). The original motion of the rotor is an out-of-plane rotation which is transformed (with the assistance of similar actuators surrounding the lens) into a vertical deflection of the lens using a complex transformation mechanism. A significant amount of the rotor torque is dissipated during the transformation of the motion. Due to the complexity of the structure of the rotor of this actuator, an undesirable tilt occurs during the translation of the lens when it is actuated by a number of similar actuators. This tilt needs a motion controller to be eliminated.
U.S. Pat. No. 8,269,395 B2 [14] discloses a large stroke rotational comb-drive actuator. It works on the repulsive force principle, and the rotor of the actuator achieves an 86 μm vertical deflection at 200 V based on a rotational stroke at each of the four edges of the actuator which is then amplified using a cantilever beam; however, it provides a low output torque due to use of an amplification mechanism and to the small area of the fingers used to generate the force.
In translational VCDs, the original motion of the rotor is a translation, and the total electrostatic force that is developed between the electrodes is directly applied to the load attached to the rotor without the use of any stroke amplification or transformation mechanism. A number of translational VCD actuators were developed. A translational VCD actuator, developed by V. Milanovic et al., achieved a translation stroke of 15 μm at 140 V. The actuator is fabricated using a Direct Reactive Ion Etching (DRIE) of an SOI wafer which enables the fabrication of large height electrodes; however, it provides a low output force as the comb electrode configuration is not area-efficient in terms of overall electrode capacitance. That is because the rotor of the actuator consists of two arrays of fingers (each array is formed along one side of the rotor). The number of fingers in these two arrays can be increased only along one direction, i.e. the lateral direction of comb fingers [11]. A self-aligned translational VCD actuator [15], developed by E. Carr et al., was able to achieve a stroke of only 1.4 μm due to the high stiffness of the supporting beams along the z-axis (out-of plane axis) and due to the low output force that can be generated by the actuator which is attributed to the area-inefficient configuration as is the case in the previous translational VCD actuators [11].
U.S. Pat. No. 6,384,952 B1 [16] discloses a translational vertical comb-drive actuator used to actuate a deformable mirror. The actuator has a cavity-tooth configuration which enables achieving a wide area for the electrodes, and it provides an out-of-plane translation of 20 μm at 100 V; however, the actuator provides only 1 degree of freedom (DOF) motion, i.e. vertical translation. The differences in the translations of the VCD actuators beneath the mirror surface result in a bi-axial rotation of the mirror surface. In other words, the comb-drive actuators have only 1-DOF motion which is a translation along the z-axis, whereas the mirror surface itself has a 3-DOF motion, i.e. translation along the z-axis, and bi-axial rotation about the in-plane axes (x and y). A limitation of this actuator is that the tooth-cavity configuration requires the rotor and the stator of the actuator to be fabricated separately. The fabricated rotor and stator wafers are then bonded together which may lead to a misalignment of sub-microns size between the upper and lower electrodes. This misalignment limits the stroke of the actuator. The cavity-tooth configuration also leads to gas damping effect between the comb electrodes as gas is trapped between the tooth and the corresponding cavity during motion of the actuators. This trapped gas has only one outlet (exit) during the actuation, which is the gap between the moving and fixed fingers. This gap is usually very small in size as compared to the finger width.
U.S. Pat. No. 7,538,471 B2 [17] discloses a vertical comb-drive actuator configuration that provides an increased rigidity to the optical surface. The goal of the invention is to overcome the problem of optical surface deformation that ensues from the deposition of a reflective metal such as gold or aluminum on the optical surface to enhance its reflectivity. The invention eliminates this problem by reinforcing the comb electrodes beneath the reflected surface in more than one direction. The actuator provides 3-DOF motion, i.e. translation along the z-axis and bi-axial rotation about the in-plane axes (x and y), without the use of any stroke amplification mechanism. It also provides a considerable large output force due to the ability of the electrode configuration to be expanded in more than one dimension. The actuator is fabricated using a surface micromachining process in which the height of the comb electrodes is limited due to the nature of the layer deposition process. These layers cannot be of a large height (thickness), which leads to a limitation on the out-of-plane translation of the actuator. In addition, the comb electrodes have a tooth-cavity configuration that contributes to appreciable damping effects, similar to the issue listed in conjunction with U.S. Pat. No. 6,384,952 B1.
A recent U.S. Pat. No. 8,711,495 discloses a MEMS autofocus mechanism that utilizes three or more translation vertical comb-drive actuators to achieve autofocus in phone cameras. The goal of this invention is to increase the resistance of MEMS Autofocus structure to shocks that occur during the drop test of the mobile phone. The drawbacks of this actuation mechanism include inefficient area-electrode layout, as it utilizes single array comb-drive actuators distributed around the lens, meaning a higher driving voltage is required; limited out-of-plane translational stroke, as the maximum height (thickness) of the electrodes is 20 microns; and low resonant frequency, as the supporting beams have to buckle during the loading of the lens to the central ring to provide an offset between the comb fingers.
In summary, the prior art translational (piston-style motion) VCD actuators have limited performance as they are unable to achieve simultaneously a large output force and a large stroke due to one or more of the following reasons:
(1) Inefficient electrode configurations of the conventional VCD actuators in which the comb fingers have an array-style structure. This structure allows multiplying the number of the fingers only in one dimension along the lateral axis of the fingers; therefore, it leads to generating a low output force. In other words, the comb fingers are essentially free-end cantilevers; hence they cannot be largely elongated along the longitudinal axis to increase the output force. Therefore, the output force can be increased by multiplying the comb fingers only along the lateral axis of the comb fingers.(2) Bonding misalignments between the rotor and stator electrodes might arise if a translational VCD actuator with a cavity-tooth configuration is fabricated using a bulk micromachining fabrication process.(3) Significant damping effects in the cavity-tooth configuration of the comb electrodes used in a number of designs that limit the bandwidth of the actuator.(4) Surface micromachined VCD actuators are limited in terms of being able to provide a large translational (piston-style) stroke. This limitation is due to the inability of surface micromachining processes for depositing large height (thickness) layers.